The present invention relates to modulation systems for transmission of digital data over a channel.
The present invention relates to digital transmission systems using single carrier modulation. The method of the present invention is compatible with shaping, and it permits any type of precoding and coding. This invention may be implemented in a system that incorporates none, some, or all of the above techniques.
Shaping is a method of selecting a signal point sequence from a subset of all allowable signal point sequences based on the respective powers of the candidate sequences. The objective of shaping is to achieve a nonuniform, Gaussian-like distribution over an expanded constellation, so as to reduce the average signal power at the same data rate. This power reduction is called shaping gain. Trellis shaping is one of the well-known shaping techniques in which the transmitter selects a sequence by a search through the trellis diagram of a convolutional shaping code, and wherein the set of all candidate sequences is not a simple Cartesian product of finite-dimensional regions.
Trellis shaping for modulation systems is described in U.S. Pat. No. 5,150,381 to Forney, Jr. et al.; in Forney xe2x80x9cTrellis Shapingxe2x80x9d IEEE Trans. Inform. Theory, vol. 38 no. 2, pp. 281-300, March 1992; and in Eyubo{haeck over (g)}lu et al. xe2x80x9cTrellis preceding: combined coding, precoding and shaping for intersymbol interference channels,xe2x80x9d IEEE Trans. Inform. Theory, vol. 38, no. 2, pp. 301-314, March 1992.
Another shaping method is described in Fischer et al. xe2x80x9cDynamics limited preceding, shaping, and blind equalization for fast digital transmission over twisted pair lines,xe2x80x9d IEEE J Select. Areas on Commun., vol. 13, no. 9, pp. 1622-1633, December 1995.
Shaping methods use an expanded signal constellation in which the symbols have a plurality of representations. This expansion increases the peak-to-average ratio of the samples generated. The ratio may be limited by employing some peak constraints, such as selecting output samples whose absolute value is less than a predetermined threshold value. The samples generated by the shaping method are further processed by the transmitter, and the peak-to-average value of the transmitted signal is usually higher than that of the samples generated by the shaping method.
Precoding is another practicable (nonlinear) processing in digital transmission systems operating over InterSymbol Interference (ISI) channels. In this scheme, the channel response has to be known at the transmitter. A precoder performing precoding, eliminates the need for a Decision Feedback Equalizer (DFE) at a receiver, and thus has two prominent advantages.
1. As operation of a DFE requires zero delay decisions, and whereas any coding scheme entails some delay in the decision path, channel coding cannot be directly combined with a DFE. Thus preceding enables the use of coding over ISI channels in the same manner as for channels without ISI.
2. Error propagation that is ascribed to the DFE is avoided in the alternative scheme that utilizes a precoder.
There are numerous ways to combine precoding with shaping. One of these combinations is known as trellis preceding which combines trellis shaping and precoding.
The peak-to-average ratio of the transmitted signal plays a major role in the design of analog circuitry and has a direct impact on the complexity and power consumption of this circuitry. In a transmitter, when the signal amplitude exceeds the maximum input value of a D/A converter, the output signal is typically clipped to a threshold level which results in a distorted transmitted signal, and degrades the system performance. Conversely, the design of a system that accommodates a high peak-to-average ratio entails the use of high power analog drivers. Numerous methods for reducing the peak-to-average value of the transmit signal of multicarrier transmission schemes have been proposed where this problem is most critical, for example in the following:
A. E. Jones et al., xe2x80x9cBlock coding scheme for reduction of peak to mean envelope power ratio of multicarrier transmission schemes.xe2x80x9d Electronics Letters, vol. 30. no. 25, pp. 2098-9, December 1994;
S. J. Shepherd et al., xe2x80x9cSimple coding scheme to reduce peak factor in QPSK multicarrier modulation.xe2x80x9d Electronics Letters, vol.31. no.14, pp. 1131-2, July 1995;
U.S. Pat. No. 5,623,513 to Chow et al.; and
G. Mestdagh and P. M. P. Spruyt, xe2x80x9cA method to reduce the probability of clipping in DMT based transceivers.xe2x80x9d IEEE Trans. Commun., vol.44, no.10, pp.1234-8, October 1996.
These methods are based on processing blocks of digital data, and hence cannot be used in systems that employ single carrier modulation which operates sequentially.
Viterbi search methods are described in:
A. J. Viterbi, xe2x80x9cError bounds for convolutional codes and an asymptotically optimum decoding algorithm.xe2x80x9d IEEE Trans. Inform. Theory vol.IT-13, pp.260-9, April 1967; and
D. G. Forney, Jr., xe2x80x9cThe Viterbi Algorithm.xe2x80x9d Proc. IEEE vol.61, pp.268-78, March 1973.
The disclosures of all publications mentioned in the specification and of the publications cited therein are hereby incorporated by reference.
The present invention seeks to provide transmission systems incorporating one or both of shaping and preceding. The proposed method is also compatible with coding. A preferred embodiment of the present invention generates a plurality of signal point sequences and selects one sequence for transmission. This selection is carried out in a manner tending to reduce the peak power of the transmitted signal. This selection can also be carried out in a manner tending to minimize both the peak power, and the average power, of the transmitted signal. Methods that generate a plurality of candidate signal point sequences, and select one sequence for transmission based on the average powers of the candidate sequences were originally proposed for shaping. However, in the present invention, the selection criteria is modified. In order to realize these selection criteria, the present invention also includes means for predicting the level of the final signal at the output of the transmitter when each one of the candidate sequences is selected for transmission and routed for further processing by filters and other units included in the transmitter.
There is thus provided in accordance with a preferred embodiment of the present invention a method for mapping digital input data, including a sequence of input samples, into an interim signal point sequence, for use in conjunction with a single carrier modulation transmitter, the method including generating a set of candidate signal point sequences characterized in that the original digital input data is recoverable from a final signal, for each input sample, selecting an interim signal point sequence, into which to map the sequence of input samples, from among the set of candidate signal point sequences, so as to obtain a low peak power value of a final signal generated from the interim signal point sequence by the signal processing procedure, relative to the peak power values of the final signals generated respectively by other sequences within the set of candidate signal point sequences by the signal processing unit, and using a single carrier modulation transmitter to transmit the final signal generated from the interim signal point sequence, sample by sample, to a receiver, thereby to reduce the peak power value of the final signal generated from the interim signal point sequence for each individual input sample in an input data sequence, relative to the peak power value of a final signal generated from a single, fixed signal point assigned to the individual input sample in the input data sequence, wherein the final signal is composed of a sequence of samples each derived from the interim signal point sequence selected for a corresponding input sample, and wherein the original data is recoverable by a receiver without knowledge as to the identity of the interim signal point sequence selected for each input sample.
Further in accordance with a preferred embodiment of the present invention the method includes, at a receiver, receiving and recovering the original digital input data without using information regarding the selecting step.
Still further in accordance with a preferred embodiment of the present invention the selecting step is also operative to select the interim signal point sequence so as to obtain a relatively low average power value of the final signal generated from the interim signal point sequence by the signal processing procedure, relative to the average power values of the final signals generated respectively from other sequences from among the set of candidate signal point sequences by the signal processing procedure, thereby to reduce the average power value of the final signal generated from the interim signal point sequence for each individual input sample in an input data sequence, relative to the average power value of a final signal generated from a single, fixed signal point assigned to the individual input sample in the input data sequence, for any given signal processing procedure.
Additionally in accordance with a preferred embodiment of the present invention the selecting step is also operative to select the interim signal point sequence so as to obtain a relatively low average power value of the interim signal point sequence, relative to the average power values of the other sequences from among the set of candidate signal point sequences, thereby to reduce the average power value of the interim signal point sequence for each individual input sample in an input data sequence, relative to the average power value of a single, fixed signal point assigned to the individual input sample in the input data sequence, for any given signal processing procedure.
Further in accordance with a preferred embodiment of the present invention the selecting step is based at least partly on an analysis of the average power value of the final signal generated from the interim signal point sequence by the signal processing procedure.
Still further in accordance with a preferred embodiment of the present invention the selecting step is based at least partly on an analysis of the average power value of the interim signal point sequence.
There is also provided in accordance with another preferred embodiment of the present invention, a system for mapping digital input data, including a sequence of input samples, into an interim signal point sequence, for use in conjunction with a single carrier modulation transmitter, the system including a candidate set generator operative to generate a set of candidate signal point sequences characterized in that the original digital input data is recoverable from a final signal, and an interim signal point sequence selector operative, for each input sample, to select an interim signal point sequence, into which to map the sequence, from among the set of candidate signal point sequences, so as to obtain a low peak power value of a final signal generated from the interim signal point sequence by the signal processing procedure, relative to the peak power values of the final signals generated respectively by other sequences within the set of possible signal point sequences, by the signal processing unit, wherein the final signal is composed of a sequence of samples each derived from the interim signal point sequence selected for a corresponding input sample, and wherein the original data is recoverable by a receiver without knowledge as to the identity of the interim signal point sequence selected for each input sample.
Additionally in accordance with a preferred embodiment of the present invention the system also includes a single carrier modulation transmitter operative to transmit the final signal generated from the interim signal point sequence, sample by sample, to a receiver.
Further in accordance with a preferred embodiment of the present invention the system also includes a receiver, receiving and recovering the original digital input data without using selection information from the sequence selector.
Still further in accordance with a preferred embodiment of the present invention the selecting step includes employing a set of prediction filters to predict an output signal level of the signal processing procedure if each of the candidate signal point sequences in the candidate sequence set is selected, and wherein selection of an interim sequence is at least partially based on the output of the prediction filters.
Additionally in accordance with a preferred embodiment of the present invention the set of candidate signal point sequences is associated with a finite state trellis diagram.
Further in accordance with a preferred embodiment of the present invention the selecting step includes a search through the trellis diagram.
Still further in accordance with a preferred embodiment of the present invention the selecting step includes a Viterbi-algorithm type search.
Additionally in accordance with a preferred embodiment of the present invention the system also performs precoding.